生物学英语教程10 18 19 20 23 24原文及翻译Word文档下载推荐.docx
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生物学英语教程10 18 19 20 23 24原文及翻译Word文档下载推荐.docx
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Virusesareverysmallentities,ranginginsizefrom0.02to0.3microns.Unliketheorganismsmakingupthefivetaxonomickingdomsofthelivingworld,virusesareacellular,theydon’tconsistofcellsandconductenergymetabolism-theydon’tproduceATPandincapableoffermentation,cellularrespirationorphotosynthesis.Asforthese,thequestionsofthevirusesoriginarise.Dotheyrepresentaprimitive‘nearlyliving’stageintheevolutionoflife?
Oraretheyorganismswhichhavelostallcellularcomponentsexceptthenucleus?
Couldvirusessimplybefragmentsofgeneticmaterialderivedfromcellularorganisms?
Noonereallyknowstheanswerstothesequestions,butwedoknowthatviruseshavebeenaroundforalongtime,andthatalmosteveryformoflifeissusceptibletoviralattack.
Thebasicunitsofavirusconsistofnucleicacidsurroundedbyacapsidorcoat,composedofoneoratmostafewkindsofproteins.Theseproteinsaresoassembledastogivethevirionacharacteristicshape.Astheybudthroughhostcellmembranes,manyanimalvirusesalsoacquireamembraneconsistingoflipidsandproteins,andmanybacterialviruseshavespecialized‘tails’madeofprotein.TheviralnucleicacidisusuallyasinglemoleculeandmaybecomposedofeitherDNAorRNA,butnotboth.DNAorRNAcanbedouble-strandedorsingle-stranded.
Virusesareobligateintracellularparasite,thatiswhytheymustdependuponspecifichostsfortheirreproductionanddevelopment.Thecellsofanimals,plantsandbacteriacanallserveashoststoviruses.Animalvirusesattachtospecialsitesontheplasmamembraneofthehostcellandarethentakenupbyendocytosis.Agivenviruscaninfectonlythosecellsthathaveareceptorsiteforthatvirus.Afterthemembranebreaksdown,theviralproteincapsidisbrokendownbycellularenzymesbeforetheviralnucleicacid,inaddition,theviralnucleicacidservestodirectthesynthesisofnewcapsidproteinbytheprotein-synthesizingsystemofthehost,andthecapsidcombinewithnewviralnucleicacidspontaneously;
andinduecourse,thenewvirionsarereleasedbythehostcell.
Plantvirusesandbacteriophagesmustgetthroughacellwallaswellasthehostplasmamembrane.Infectionofaplantusuallyresultsfromattackbyavirion-ladeninsectvector.Theinsectusesitsproboscistopenetratethecellwall,andthevirionstheescapefromtheinsectintotheplant.Bacterialvirusesareoftenequippedwithtailassembliesthatinjectthenucleicacidintothehostbacteriumwhiletheproteincoatremainsoutside.Onceinsidethehostcell,thevirusgenestakesoverthemetabolicmachineryofthecellandgeneratetheirown.
Sometimes,viralDNAdoesnotimmediatelytakecontrolofthehostmetabolism,butinsertitselfintothehostchromosomeandpresentas‘silent’provirusuntilthehostcellisexposedtosomeenvironmentalinsult,suchasultravioletlightorradiation.
IftheviralnucleicacidisRNA,replicationneedsspecialenzymestomaketheprocessofRNA-to-RNAsynthesisoccurs.SomeRNAvirusescalledretrovirusesdonotcarryoutRNA-to-RNAtranscription.Instead,theirRNAistranscribedintoDNAisimmediately,thisreactioniscatalyzedbyreversetranscriptase,thennewlyformedDNAisinsertedintohostDNAandthentranscribedintoRNAandatlastnewvirusesareproduced.
Afterreplicationandcombination,mostvirusesarereleasedbylysisofthehostcell.Butinothercases,likethatoftheretroviruses,virusesarereleasedbyextrusion,aprocesssimilartobuddingtherebythevirusbecomesenvelopedinasmallpieceofcellmembraneasitmovesoutofthecell.Lysisresultinthedestructionofthecell,butextrusionallowsthecelltoremainaliveandcontinuetoproducenewvirusesforalongperiodoftime.
Acommonwaytoclassifyvirusesistoseparatesthemfirstonthenatureofthenucleicacidcomponent(DNAorRNA)andthenonwhetherthenucleicacidinthevirionissingle-ordouble-stranded.Furtherlevelsofclassificationdependonsuchfactorsastheoverallshapeofthevirusandthesymmetryofthecapsid.Mostcapsidmaybecategorizedashelical,icosahedralandsoon.Anotherlevelofcategorizationisbasedonthepresenceorabsenceofmembranousenvelopearoundthevirion;
stillfurthersubdivisionreliesoncapsidsizeandothercriteria.
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Withinacell,energyisneededateverystagetodrivethereactionsthatkeeplifeinnormalstates.Ontheearth,almostalltheenergythatfuelslifetodaycomesfromthesunandiscapturedintheprocessofphotosynthesisbyplants.Mostnonphotosyntheticorganismsobtainenergybyingestingphotosyntheticorganismsorothersthathavethemselvesingestedphotosyntheticorganisms,andtheenergystoredbyphotosynthesisisusuallyreleasedthroughaprocessknownasrespiration.Inthischapter,weshalldiscussthesetwoprocesses.
Photosynthesisisalogicalstartingpointforourdiscussionofthebasicenergytransformationoflife.InsimpletermsphotosynthesisconsistsofthereductionofatmosphericCO2tocarbonhydratebyuseoflightenergy,withanassociatedreleaseofoxygenfromwater.Thisreactioncanbesummarizedbythefollowinggeneralizedequation.
Likemanyotherphysiologicalprocesses,photosynthesisconsistsofanumberofsequentialsteps:
①trappingoflightenergybychloroplasts;
pigmentsotherthanchlorophyll(e.gcarotenoids)playanaccessoryroleinphotosynthesisbytransferringenergytochlorophylla.②splittingofwaterandreleaseofhigh-energyelectronsando2.③electrontransferleadingtogenerationofchemicalenergyintheformofATPandthereducingpowerasNADPH2.④terminalstepsinvolvingexpenditureofenergyofATPandthereducingpowerofNADPH2tofixCO2molecules,andfinallyconvertthiscompoundsintomorecomplexcarbohydrates,suchassucrose,starch,celluloseandsoon.
Carbondioxideisanexceedinglyenergy-poorcompound,whereascarbohydratesisenergy-rich.Photosynthesis,then,convertslightenergyintochemicalenergy.Inchemicalterms,theenergyissaidtobestoredbytheadditionofonemoreelectron-storesenergyinthesubstancebeingreduced.
Although,photosynthesiscanoccurinanychlorophyll-containingpartsoftheplant,leavesthatexposethegreatestareaofgreentissuetothelightaretheprincipalorgansofphotosynthesis.Throughamicroscopeitcanbeseenthattheoutersurfacesoftheleafhavealayerofepidermis,whichiscoveredbywaxylayerofcuticle.Theregionbetweentheupperandlowerepidermisconstitutesthemesophyllportionoftheleaf.Thecellsofmesophyllcontainmanychloroplasts,whichistheorganellesthatphotosynthesistakesplace.TheCO2requiredforphotosynthesiscanenterthroughsomeholescalledstomatabetweenthespacesofmesophyllcell.Chloroplastisboundedbytwoconcentricmembraneandathirdsetofinternalmembranesthatformaseriesofflattened,interconnectedsacsknownasthylakoids,wherechlorophyllmoleculesandmostoftheelectron-transport-chainmoleculesarelocated.Thelightreaction,inwhichlightenergyistrappedandconvertedintochemicalenergy,takesplaceonthethylakoidmembrane.Thedarkreaction,inwhichCO2isreducedtocarbohydrates,occursinthemorefluidstromathatsurroundsthethylakoidsacs.
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Itisevidentthatthephenomenonofinheritanceandvariationisofuniversalimportantinthelivingworld.Tounderstandit,wemustexplorehowhereditarymaterialexpressesitselfinnewcombinations,andwhetherprinciplescanbeformulatedaboutsocomplexevent.Thisexplorationisthestudyofthebranchofbiologyknownasgenetics.
SomeofthebasicconceptsofhereditygrewoutofexperimentsperformedbyGregorMendelinthemid-1800s.MendelspentmostofhislifetimeasamonkinanAustrianmonasteryandduringthistimehecultivatedgardenpeas,anddidaseriesofexperimentstostudyinheritanceinplants.Wenowknowthatowingtohiselaboratedesign,uniquetechnique,MendelformulatedtwoexcellentLawsofInheritancefromhisbreedingexperimentsonthegardenpeas:
oneistheLawofSegregation,theotheristheLawofIndependentAssortment.
TheLawofSegregationemphasizeonsingletraitsanditcanbestatedasfollows:
theinheritanceofeachindividualtraitisdeterminedbyhereditaryfactors(genes),eachorganismpossessestwoinheritancefactorsforeachcharacter,whengametesareformed,thetwofactorsseparateintoseparatedgametes.Anoffspringformedbythefusionoftwogametesthereforereceivesonefactorforeachcharacterfromeachparent.TheLawofIndependentAssortmentinvolvingtwoormoretraitsandstatesthatduringgametesformation,whentwoormoregenesareinvolvedinacross,theallelesofonegeneareinheritedindependentlyoftheallelesofanothergene.Hisgreatachievementlaidasolidfoundationforthegenetics.Inordertofurtherunderstandingofthesetwolaws,somedetailscanbeknowneither:
1.Hereditarytraitsarecontrolledbydiscreteunitsthatpassunchangedfromgenerationtogeneration.Forexample,thetraitwhiteflowerseemstodisappearintheF1generation,butreappearsontheF2progeny,andthattherearenointermediatecolors,onlyredorwhite.
2.Eachtraitisproducedbytwohereditaryfactors.Thisisanecessaryassumptiontoaccountforthewayinwhichatraitsuchasflowercolorappearsinsuccessivegenerationsinapredictableratio.
3.Whentwocontrastinghereditaryfactorsarepresentin
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